The present invention relates to the general field of transmitting information from a salt-cavern formed in the ground to the surface. More precisely, the invention relates to transmitting information collected at any height within a salt-cavern while still enabling the cavern to be operated normally (filled, tapped, etc.).
Salt-caverns are generally used for underground storage of hydrocarbons such as natural gas or oil. Such hydrocarbon storage can be necessary for retaining energy availability during a crisis (so-called “strategic” storage) or for making it possible to accommodate seasonal peaks in consumption (so-called “seasonal” storage).
Conventionally, a salt-cavern is obtained by drilling a borehole through geological formation beds (rock salt) and by washing out salt with a flow of fresh water in order to create a cavern of desired shape and volume. A production tube is lowered to the bottom of the cavern to enable it to be filled with hydrocarbon.
When storing natural gas, it is essential to monitor continuously the physical parameters internal to the cavern (pressure, temperature, available volume, etc.) while it is in operation, i.e. throughout the period in which the cavern is being filled, is at rest, or is being tapped. In particular, its internal pressure must remain firstly slightly greater than the pressure of the formation in order to avoid any risk of subsidence and loss of useful volume by salt creep, and secondly below the pressure at which the rock fractures in order to guarantee that the cavern remains leaktight. In addition, the volume of gas contained in the cavern depends strongly on storage pressure, and increasing storage pressure even by only a few millibars can lead to several hundreds of thousands of additional cubic meters of gas being stored. Under such conditions, continuous monitoring of pressure while the cavern is being filled makes it possible to determine accurately the volume of gas to be stored.
At present, these physical parameters are calculated from measurements made at the head of the borehole. However, the information that such measurements can give about the situation at the bottom of the cavern is only approximate, thereby leading to large errors in predicting storage.
It is also known to introduce measurement sensors into the annular space defined between a central operating column and the cylindrical wall of the borehole, which sensors are connected to the surface by electric cables. Nevertheless, that technique can be applied to existing boreholes only after implementing expensive modifications. In addition, such measurements performed in the borehole differ from measurements performed in the cavern.
In order to measure these parameters in the cavern, another solution consists in suspending measurement devices from an electric cable connected to the surface. However, in order to ensure that the cable connecting the measurement devices to the surface is not cut, the valves closing the borehole need to be kept in an open position while measurements are being taken. That solution therefore raises obvious problems of safety, and prevents any tapping operations from being performed since there would be a risk of the cable and the measuring devices being entrained therewith.